Cleaner Cooking with Gas

Abstract

Many chefs prefer cooking with gas, a fuel long promoted as clean burning. But growing concern over indoor air pollution threatens to dirty the gas range's formerly pristine image. So today appliance designers are at work to develop less-polluting burners. Already there are at least two promising candidates on the horizon, both due to be field tested this year. The kitchen stove is unique among modern gas appliances in that its combustion products are emitted directly into the home. Gas is used for cooking in about 45 percent of U.S. homes, and studies show most of these homes don't vent outdoors the pollutants their stoves emit. Those pollutants include carbon monoxide (CO), carbon dioxide (CO2), nitric oxide (NO), nitrogen dioxide (NO2) and aldehydes such as formaldehyde. There is some controversy about what health risks, if any, those emissions pose (see sidebar). But not waiting for definitive findings that exonerate or convict gasrange emissions of affecting health, the Gas Research Institute (GRI) in Chicago has begun sponsoring development of low nitrous-oxide (NOx for NO and NO2) range burners. A new flame insert developed for GRI at the American Gas Association (AGA) Laboratories in Cleveland indeed makes gas ranges cleaner. A burner fitted with one of these inserts and turned on high will cut its NOx emissions more than 40 percent. At low burner settings the insert reduces NOx even better. Designed to fit onto the head of existing range burners, it consists of a pair of stainless-steel rings roughly three inches in diameter and mounted an eighth of an inch apart, one atop the other. Placed over a conventional burner, the rings sit about a quarter inch out from the portals through which flames exit the burner. When the burner is on, its flames are forced to pass between the rings, transferring some of their heat. This cools the flame's peak temperatures by distributing heat more uniformly. when you cool the flame you reduce the NOR, explains Douglas DeWerth, who with Bill Sterbik has designed the model under test. The rate of the chemical reaction that forms nitrogen oxides is tied to temperature: the hotter the flame, the more oxygen and nitrogen react. However, a cooler flame also tends to increase CO levels. CO is a natural combustion product, but if the flame is hot enough for long enough, DeWerth says that CO will burn over to CO2. When a flame's temperature is reduced, however, the chemical reaction that drives the conversion of CO to CO2 slows up faster than the rate of CO formation. And that causes levels of CO to build up. In some early tests, NO, reduction was accompanied by significant CO production 6,000 parts per million (ppm) or more for an increase of more than 10-fold over normal burner-emission levels and a factor of 7 higher than that recommended by the American National Standards Institute. Before the insert was added, burners used in the AGA Laboratories test produced about 560 ppm CO (measured at the burner, not in room air). With the insert, CO increased to 650 ppm. However, NO2 production at the burner fell 28 percent and total NO, dropped 42 percent (from 103.3 ppm). Magic Chef, Inc., a range manufacturer, is supplying ranges to AGA Labs for burner modification and advice on design adaptations to encourage consumer acceptance. One suggestion that has been incorporated is use of a four-inch semi-opaque cap to hide the ring system (considered too ugly to display prominently). While the disk, made from Vycor, an impact-resistant high-silica glass, does not affect emissions, it has reduced the efficiency of modified burners four or five percent, DeWerth says. It's also the insert's most expensive component. We're just about finished with our [laboratory] evaluations of a prototype range the manufacturer sent us-where all four top burners have been modified, DeWerth told SCIENCE NEWS. Within a few months he hopes to see Magic Chef place four to six modified ranges in homes to field test them under real-life conditions -such as spills and impact with heavy cast-iron pots. If all goes smoothly, the inserts might be ready to market by the end of the year. However, don't expect to see commercial models any time soon. As DeWerth put it, what incentive does a manufacturer have to develop ranges whose burners cost more, offer lower efficiency and are potentially less attractive than those of its competitors particularly when there is still a question of whether existing emission levels harm health? In fact, the very presence of the inserts could serve to escalate debate over gas-range safety, something manufacturers are understandably loath to do. Then why were these inserts developed? We're worried about the future because we see what the Consumer Product